Slang term potheads use for getting high. Especially when they are really looking forward to it. It has become proper stoner etiquette to not just say they are about to take part in an illegal substance, but to use a term like this to try and make it sound nice and neat.

1. Rhonda wrote on her Twitter account: "After I finish this housework I'm going to have a 420 moment."

2. First Brain Cell: "Well, it looks like its down to just you and me."
Second Brain Cell: "Yeah, but if she has one more 420 moment... we're history!"